With the constant increase of mobile data services, the 3rd Generation Partnership Project (3GPP) organization has developed long-term evolution (LTE) specifications and LTE-Advanced (LTE-A) specifications and the LTE has been rapidly deployed by a number of operations worldwide. As the next generation cellular communication standard, the LTE or LTE-A system can operate in both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode.
The LTE with TDD mode may also be called as a TD-LTE system. Due to channel reciprocity, the TD-LTE system has attracted more and more interests to obtain downlink beamforming gains with a large number of transmitter antennas. One of typical antennas deployments is to employ eight physical transmitter antennae on eNB side due to its excellent uplink receiving performance and relative good downlink beam forming performance. For the 8-antenna deployment, one of the most interesting topics is how to make such macro outdoor 8-antenna cover indoor users which generate most of LTE traffic. This lies in that the TD-LTE system employs a relatively higher carrier frequency, which could provide a larger bandwidth but a worse coverage than traditional GSM system, whereas building dedicated IBS (In building system) for indoor LTE converge is very expensive.
In outdoor-to-indoor scenario, to enable user equipment (UE) to access networks with extreme weak signal, for poor coverage points, the operator normally would set a basic requirement to ensure the UE to receive and correctly demodulate/decode the uplink and downlink common signals and channels. However, the basic requirement correctly demodulating/decoding constrains the cell coverage.
Generally, in TD-LTE system with 2-antenna, the cell coverage bottleneck is the uplink common signaling channel PRACH format 0 (i.e., random access channel) due to the UE power limitation, as illustrated in FIG. 1A. However, in TD-LTE system with 8-antenna, the downlink common signaling channels become the bottleneck for the cell coverage, just as illustrated in FIG. 1B. This lies in that the network side needs to send out the downlink common signaling channels to all UEs in a broadcasting way and thus more downlink transmission antennas can not provide more gains when the same transmission power is used.
To address the cell downlink coverage problem of the TD-LTE system with 8-antenna, two ways may be employed, i.e., increasing total downlink transmission power and performing power boosting on downlink common signaling channels. The increasing of total downlink transmission power is difficult to be implemented because required total output power is too high to be supported by current products due to extremely high component cost. The power boosting approach is a practical method by a flexible power allocation between the common signaling channels. The power boosting may be performed on the CRS or downlink common channels. For a purpose of illustration, the flexible power allocation for downlink common channels and the common reference signal (CRS) resource elements (REs) is illustrated in FIG. 2, wherein CRS REs occupy specific fixed RE within each downlink time-frequency grid while the common channels would change according to the scheduled PDSCH loading.
As mentioned hereinabove, an option of power boosting approach is to increase power of CRS signals. The CRS signals are used by UEs to estimate downlink radio channels for coherent demodulation of downlink physical common channels. Furthermore, they are also used by UEs to acquire channel-state information (CSI) initial cell-selection and handover decisions. Thus, increasing the CRS transmission power can be helpful for UEs to obtain the better channel estimation, hence beneficial to the demodulation of all downlink common physical channels. However, in accordance with 3GPP specifications, the CRS REs power setting is carried by a system information type 2 with the information element referenceSignalPower. The system information type 2 has an updating periodicity of about 100 ms, which is too long to flexibly improve the cell coverage. This means the power of CRS REs can not be set flexibly with the traffic variation.
The other option is to perform downlink common channel power control. The increasing of the transmission power of downlink common channels may directly benefit these channels' demodulation due to increased power of received signal. However, if the downlink common channels are highly loaded especially in the cell edge, the network side has to increase the transmission power on a large fraction of downlink physical resource elements, which is hard to be supported by current terminal device due to a large power requirement.
Besides, in Chinese patent application publication No. CN102118842A titled “Downlink power control method and device applied to LTE system,” there is proposed a downlink power control solution in a LTE system. In the proposed solution, power of the CRS signals are first determined, then based on the determined power for the CRS signals, power of the downlink physical channels are determined. It is clear that the solution proposed in the Chinese patent application publication also belongs to a downlink common channels power control approach as mentioned hereinabove and similarly it is involved in similar problems as well.